Dry film resists have been widely used recently for making printed plate boards.
In particular, it has been well known to use a polymer binder having a carboxy group for a dry film resist composition, making it possible to treat the dry film resist with an aqueous alkali solution.
Also, the use of various photopolymerization initiators for dry film resists has been proposed for forming pattern by a photographic process. Among them, lophine dimers described in JP-B-45-37377 and 48-38403 (corresponding to U.S. Pat. No. 3,549,367) (the term "JP-B" as used herein means an "unexamined published Japanese patent application"), JP-A-56-35134 (corresponding to U.S. Pat. Nos. 4,311,783 and 4,252,887) (the term "JP-A" as used herein means an "examined published Japanese Patent Application"), and U.S. patent application Ser. No. 07/391,819 give photopolymerizable compositions having a high sensitivity.
However, when a photoresist using the lophine dimer as a photopolymerization initiator is laminated on a copper layer-applied laminated plate, storage for a long period of time often results in the discoloration of the surface of the copper layer at the unexposed portions revealed by the development into reddish brown (so-called discoloring) and the formation of a development residue (or scum). Although this tendency increases depending on the length of time the laminate is stored, in practice the laminate is allowed to stand for several days before use. Discoloring and the formation of development residue are serious problems since they frequently cause etching residues and plated layer peeling.
To prevent discoloring and/or the formation of development residue, various additives for photopolymerization systems have been proposed. For example, JP-B-58-22486 and JP-A-61-l94438 describe the use of low-molecular weight carboxylic acids such as citric acid, malonic acid, etc. In particular, JP-A-61-l94438 discloses that the use of low molecular weight carboxylic acids having a molecular weight of not more than 300 is effective.
The addition of these carboxylic acids can reduce discoloring and the formation of development residue. However, if the amount of the carboxylic acid added is too low, a sufficient effect is not obtained, and if a large amount of the carboxylic acid is added to obtain a sufficient effect, the sensitivity of the resist and the alkali resistance of the resist image are reduced. Thus, these carboxylic acids are insufficient as a discoloring inhibitor.
On the other hand, JP-A-62-287240 discloses the use of a compound having a (meth)acryloyl group, an acid value of from 30 to 300, and a weight average molecular weight of from 200 to 2,000, together with a methacrylic acid ester copolymer having a carboxy group on the side chain and a weight average molecular weight of from 100,000 to 400,000 and a photopolymerization initiator for improving the etching resistance of a resist ink composition for etching.
However, the object of that invention is to improve the etching resistance, and there is no description in the specification thereof about the effect of inhibiting discoloring. And, all the compounds described therein have a molecular weight of 300 or less. Furthermore, when a copper plate having the photopolymerizable composition using the compound described in the aforesaid paten specification laminated thereon is stored for a long period of time, granular solid matter forms on the surface of the copper plate. Such solid matter is not removed by development processing and causes serious defects in the etching step and the plating step.